The present invention relates to a time-division multiplexing optical signal transmission network and its switching node according to line switching system, packet switching system, ATM switching system or the like.
Regarding an apparatus having optical distribution function in the prior art, as discussed in the spring symposium in 1990, C-220 of the electronics, information and communication engineers of Japan, a no-load 1.times.4 optical gate switch constituted by five laser diode optical gate submodules and a 1.times.4 optical branch waveguide circuit is reported. Switching laser diode optical gate submodules are connected respectively to four output ends of the optical branch waveguide circuit, and one submodule for polarization compensation is connected to input end so that it has an activation layer surface vertical to other four gates. The submodule has structure that an optical gate of InGaAsP laser diode for 1.3 .mu.m band and a single mode top-bulb optical fiber with top end radius 10 .mu.m installed on both ends of the optical disk are enclosed within a Cu-W cabinet, and coupling loss between the laser diode gate and the top-bulb optical fiber is 3 dB. The optical branch waveguide circuit is constituted by a quartz optical waveguide/Si and has 1.times.2 branch two-stage structure, and transmission loss between the fiber optical branch waveguide circuit and the fiber is 8 dB. Size of the whole switch is 12 mm.times.75 mm. Exciting current of 27-30 mA is applied to each laser diode optical gate thereby no-loss switching is realized.
Since an optical gate switch of 1.times.(n-th power of 2) according to the same principle as that of the above-mentioned no-load 1.times.4 optical gate switch can be compensated in loss between input/output during switching by gain possessed by the contained laser diode optical gate, its application as distribution node in line switching system or a time-division multiplexing optical signal transmission network is being considered. In the above-mentioned no-load 1.times.4 optical gate switch, however, in addition to the deterioration quantity of S/N ratio (signal to noise strength ratio) in input/output side by the laser diode optical gate submodules before and after the optical branch waveguide circuit, it is further deteriorated by 8 dB due to the optical branch waveguide circuit. In general, in an optical gate switch of 1.times.(n-th power of 2), a problem exists in that the S/N ratio at the input/output side is deteriorated by (3.times.N) dB or more in principle due to the optical branch waveguide circuit. Five laser diode optical gate submodules are used in the no-load 1.times.4 optical gate switch, but laser diode optical gates of (n-th power of 2) pieces or more are necessary in the 1.times. (n-th power of 2) optical gate switch and a problem exists also in that an optical gate switch of small size and large scale cannot be easily realized. Further considering application as distribution node in the time-division multiplexing optical signal transmission network by packet switching system, ATM switching system or the like, in the optical gate switch of 1.times. (n-th power of 2), since a specific packet cannot be distributed while optical signal train as a whole is held, if this is connected in cascade, one packet cannot be distributed from the distribution nodes of two positions or more, or similar problem exists in that the use state is limited.